The current objective of our research program is to define the process by which concatemeric phage DNA is converted to mature phage particles containing highly condensed DNA molecules of precisely defined length. We are pursuing the problem by a study of the roles of genes 16, 17, 49 and 31 in the formation of the phage head. Three mechanisms of action can be predicted for the gene 31 product: (a) it prevents binding of P23 to the cell membrane by covering or altering proteins of the membrane; (b) it prevents binding of P23 or promotes its release by interacting with other capsid initiating proteins, like P22 and 1PII; (c) it prevents binding or promotes release of P23 by altering the structure of P23, yielding P23'. If P31 alters the covalent structure of P23, that change might be apparent either in the peptide fingerprint of P23', the hypothetical product, or in its isoelectric focusing pattern. Although occasionally we have detected peptide differences between the large molecular weight P23 from the various infections, the bulk of our results suggest that the peptides from all are identical. Electrofocusing of P23 indicates some heterogeneity, which is being investigated. Our studies on the role of gene 17 indicate multiple functions for that gene product. One result, with ts P75, indicates that P17 is required at late stages of capsid morphogenesis after cleavage of the capsid protein precursors but prior to DNA maturation. Another, with ts P67, indicates that P17 may be a component of the phage capsid. Since 'empty' capsids are maturable, in some hosts, when gene 17 function is restored, these capsids may be intermediates in capsid assembly and a capsid-filling role is also indicated for P17. BIBLIOGRAPHIC REFERENCES: Dewey, M., and Frankel, F.R. Two suppressor loci for gene 49 mutations of bacteriophage T4. 1. Genetic Properties, DNA synthesis Virology, 68:387-401 (1975). Dewey, M., and Frankel, F.R. Two suppressor loci for gene 49 mutations of bacteriophage T4. 11. DNA and capsid maturation. Virology, 68: 402-417 (1975).